Segment Reporting (Schedule of Gross Premiums Written Allocated to the Territory of Coverage Exposure) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 547,755	$ 625,677	$ 977,343	$ 1,159,652	$ 407,766	$ 640,269	$ 827,415	$ 922,090	$ 3,310,427	$ 2,797,540	$ 2,374,576
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									0	(7)	0
Property
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,760,926	1,440,437	1,111,263
Property | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									978,063	954,269	743,226
Property | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									464,311	305,915	210,168
Property | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									144,857	49,486	37,611
Property | Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									71,601	49,821	44,536
Property | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									66,872	48,182	55,043
Property | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									19,273	14,151	13,729
Property | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									15,949	18,613	6,950
Casualty and Specialty
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,549,501	1,357,110	1,263,313
Casualty and Specialty | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									667,125	622,757	646,381
Casualty and Specialty | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									776,976	686,253	581,972
Casualty and Specialty | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									15,296	9,752	5,541
Casualty and Specialty | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									31,734	10,104	13,840
Casualty and Specialty | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									3,667	4,141	5,073
Casualty and Specialty | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 54,703	$ 24,103	$ 10,506